The present invention relates generally to gas turbine engines, and, more specifically, to turbine nozzles therein.
In a gas turbine engine, air is compressed in a compressor, mixed with fuel and ignited in a combustor for generating hot combustion gases which flow downstream through one or more turbine stages which extract energy therefrom for powering the engine. A high pressure turbine (HPT) powers the compressor of the core engine, and a low pressure turbine (LPT) is disposed downstream from the HPT for powering a fan, for example. Each turbine stage includes a stationary turbine nozzle followed in turn by a row of rotor blades mounted to the perimeter of a rotor disk.
The HPT nozzle is mounted at the exit of the combustor and therefore receives the hottest temperature combustion gases therefrom. Accordingly, the turbine nozzle is specifically configured for channeling therethrough a portion of the compressor air which is bled therefrom for cooling the turbine nozzle for obtaining a suitable useful life thereof during operation. The nozzle vanes are hollow airfoils extending radially between outer and inner bands which support the vanes. Various conventional configurations exist for suitably cooling both the vanes and the bands, with impingement and film cooling typically being used for providing cooling of these components. To effect impingement cooling, each of the bands includes an impingement baffle in the form of a perforated plate through which bleed air is channeled for impingement against the outer surface of the bands for the cooling thereof. One or more perforated hollow inserts are suitably mounted within each of the vanes for directing impingement air against the inner surface of the vanes for impingement cooling thereof.
During operation, the hot combustion gases flow around each of the nozzle vanes between the outer and inner bands. Accordingly, the turbine nozzle thermally expands upon being heated, and contracts when temperatures are reduced. Furthermore, significant temperature gradients exist along the vanes and bands due to heating by the combustion gases and cooling by the bleed air. The temperature gradients and differential thermal movement of the nozzle components result in thermally induced strain and stress which must be kept within suitable limits for obtaining a useful life of the nozzle during operation.
Each nozzle is an annular structure with the outer and inner bands typically being formed in arcuate segments to eliminate the hoop restraint which would otherwise be caused by continuous rings. Each vane is fixedly joined at outer and inner ends thereof to the outer and inner bands by brazing or being integrally cast therewith. Continuous outer and inner bands joined by the vanes would create excessive thermal strain and stress due to differential expansion and contraction thereof providing a reduced low cycle fatigue life. Dividing the outer and inner bands into a suitable number of segments with two or more vanes per segment significantly reduces thermally induced stress, but increases the complexity of the turbine nozzle since suitable seals must then be provided between the individual nozzle segments to prevent undesirable leakage therethrough.
Even in a nozzle segment having two vanes fixedly joined to the outer and inner band segments, differential thermal movement between the components nevertheless results in thermally induced strain and stress which affects the useful life of the nozzle. However, since the bands and vanes are typically formed of the same or similar high temperature metal or material, the thermal coefficients of expansion, and thermal conductivity, thereof are correspondingly the same which reduces the thermally induced stress therefrom.
Since the overall efficiency of the gas turbine engine is directly related to the temperature of the combustion gases, engine efficiency is limited by the ability to operate the turbine nozzle at high temperature. Present nozzle materials are superalloys, such as single crystal nickel based materials, which have allowed engines to be operated at relatively high thermal efficiency. Further advances in engine efficiency require further increase in combustion gas temperature which cannot be obtained using conventional superalloys and conventional cooling techniques.
Accordingly, advanced, high temperature materials are being developed for use in turbine engines. One class of these high temperature materials is referred to as intermetallic, and includes for example nickel aluminide (NiAl) superalloys which have even higher temperature capability than that of in-service superalloys. They enjoy higher melting temperature while maintaining strength at higher temperatures. And they have substantially higher thermal conductivity which enhances cooling and reduces hot spots.
Another class of advanced high temperature material is referred to as ceramic matrix composite (CMC) which also has substantially higher temperature capabilities than conventional superalloys. CMC materials also maintain strength at relatively higher temperature than that of conventional superalloys.
However, both of these advanced materials are relatively brittle when compared to conventional superalloys, with a corresponding loss of toughness inherent therein. These materials also have substantially different coefficients of thermal expansion, or different thermal conductivities, compared to conventional superalloys. If these materials were conventionally fixedly joined to conventional nozzle outer and inner bands, they would fail prematurely during operation due to thermal stress since the restraint imposed by the bands on the relatively brittle vanes would cause them to fail upon differential thermal movement due to expansion or contraction therebetween. The differential thermal movement is caused by difference in coefficients of thermal expansion or difference in thermal conductivities during thermal transient operation. In order to effectively utilize these advanced high-temperature, but brittle, materials in a gas turbine engine, an improved turbine nozzle configuration is required.